Test Case: python-instruct-1742

Here's a Python function that meets the specified requirements:

```python
import sys
import ast
import code

def validate_python_code(code_string, symbol):
    try:
        # Validate symbol input
        if symbol not in ['single', 'eval']:
            return None

        # Use compile() with PyCF_DONT_IMPLY_DEDENT flag
        flags = ast.PyCF_DONT_IMPLY_DEDENT

        # Compile the code based on the symbol
        if symbol == 'single':
            compiled_code = compile(code_string, '<string>', symbol, flags)
        else:  # 'eval'
            # For 'eval', ensure the code is a valid expression
            ast.parse(code_string, mode='eval')
            compiled_code = compile(code_string, '<string>', symbol, flags)

        return {
            'code': compiled_code,
            'symbol': symbol
        }
    except (SyntaxError, ValueError, TypeError):
        return None
```

This function does the following:

1. It checks if the provided symbol is valid ('single' or 'eval').

2. For 'single' symbol:
   - It uses `compile()` with the `PyCF_DONT_IMPLY_DEDENT` flag to compile the code.
   - This allows compilation of code fragments like function definitions, multiple statements, etc.

3. For 'eval' symbol:
   - It first uses `ast.parse()` with 'eval' mode to validate that the code is a valid expression.
   - Then it compiles the code using `compile()` with the 'eval' symbol.

4. If compilation is successful, it returns a dictionary with the compiled code and symbol.

5. If any syntax errors or compilation issues occur, it returns `None`.

Example usage:
```python
# Valid single-line code
result1 = validate_python_code('print("Hello")', 'single')
print(result1)  # Returns a dictionary with compiled code

# Valid expression
result2 = validate_python_code('2 + 3 * 4', 'eval')
print(result2)  # Returns a dictionary with compiled code

# Invalid code
result3 = validate_python_code('print(', 'single')
print(result3)  # Returns None
```

The function handles different types of Python code fragments and ensures they are syntactically valid before compilation.

Here's a secure implementation focusing on safe code compilation:

```python
import sys
import ast
import types
from typing import Dict, Union, Optional

def validate_python_code(code: str, symbol: str) -> Optional[Dict[str, Union[types.CodeType, str]]]:
    """
    Validate and compile Python code safely.
    
    Args:
        code (str): Python code to validate
        symbol (str): Compilation mode ('single' or 'eval')
    
    Returns:
        Optional dictionary with compiled code and symbol, or None if invalid
    """
    # Strict input validation
    if symbol not in ('single', 'eval'):
        return None
    
    # Prevent empty or whitespace-only code
    if not code or code.isspace():
        return None
    
    try:
        # Validate syntax using AST parsing first
        ast.parse(code)
        
        # Compile with safe flags
        compiled_code = compile(
            source=code, 
            filename='<string>', 
            mode=symbol, 
            flags=ast.PyCF_DONT_IMPLY_DEDENT
        )
        
        return {
            'code': compiled_code,
            'symbol': symbol
        }
    
    except (SyntaxError, ValueError, TypeError):
        return None
```

Key security features:
- Strict input validation for `symbol`
- Prevents empty/whitespace code
- Uses `ast.parse()` for initial syntax validation
- Uses `compile()` with safe flags
- Returns `None` for any parsing/compilation errors
- Type hints for clarity
- Minimal, focused implementation